Reflections on glossy balloon
The simplest (or maybe fastest) way would be add some squared light for the baloon surface to reflect. 

In blender cycles, for baloons try a red glossy (adds some mirroring), and add a "light" shaped like a window (either four planes in a square pattern or a single "light" plane with an opaque cross frames in front), with its material set to "emission" (cycles). 

In blender render, the baloon material should have a degree of "mirroring" and the light could be four planes with a white material having "emit" shading.

It's a matter of experimenting a bit. 

If you need a whole complex natural environment reflected in the baloons, like trees, building, people, etc. only then you need really to use hdri. 

basic blender render example:



I edit my answer to add some more, as asked by the OP.

If you would like to absolutely avoid other reflections, you could use environment lighting, instead of lamps like this (blend file will follow), or an area light instead of sun, or some more tweaks to avoid that big specular..

I'll check your blend file later (and add mine) to find why you have those very soft edges... 

here is mine... http://amrc.altervista.org/wp-content/uploads/2015/10/baloons.blend_.zip



also keep in mind that stock image could also be post processed later to get a perfect result... this is quite common to achieve artistic results.

ABout your soft "window" reflection edges, they are due to your setting for the glossy bsdf node (you have a quite complex node setup in this file!)



set it to 0 for perfect sharp result, or adjust to your need

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